Unipolar hot-electron transistors (HETS) have received little attention when compared with the level of effort focused on bipolar transistors. The HET has long shown promise as a high frequency device, but its development has been impeded by stringent fabrication requirements, immature growth techniques, and the lack of design tools for optimizing electron-wavelength-scale devices.
In recent years, many of these difficulties have been surmounted. At 77K, resonant-tunneling hot-electron transistors (RHET) have been demonstrated with common emitter current gain on both gallium arsenide and indium phosphide substrates. See N. Yokoyama, et al., Solid State Electronics 31 (1988) 577.
In a conventional RHET, an injector structure is interposed between the emitter and the base. This injector structure includes a quantum well having a relatively low conductance band minimum electron energy level and a width that is on the order of an electron wavelength integer or half-integer multiple. The quantum well is bounded by high conductance band energy level barriers that are sufficiently thin that the electrons may tunnel through them. Electrons tunneling through the barrier from the emitter into the quantum well will resonate therein. The electron flux or current density passed through the quantum well will depend upon the energy of the electrons; electrons of certain energies will "couple" with the well better than others. Electrons resident in the quantum well are thus likely to be quasi-monoenergetic. These electrons are injected through the remaining barrier into the base.
However, the energy of injected electrons in conventional RHETs tends to be too high, and these electrons tend to be transferred from the lower-energy .GAMMA. mode to the X and L modes through internal scattering, then to be lost to the collector. Room-temperature gain for conventional RHETs has therefore been difficult to achieve.
To this date, only two instances of room temperature current gain in a HET have been reported: an InP-based RHET (T. Mori et al., Extended Abstract of the Conf. of Solid State Devices and Materials (1988) 507), and a device using AlGaAsSb heterojunctions (A. F. J. Levi and T. H. Chiu, Applied Physics Lett. 51 (1987) 985). However, no InP-based RHETS have been reported to operate with gain better than 2.5 at room temperature.